1. Field of the Invention
My present invention relates generally to artificial cardiac pacemakers, and more particularly to an implantable cardiac pacemaker which automatically adjusts pulse parameters in response to detected physiologic or environmental conditions.
2. Relevant Background
Early implantable cardiac pacemakers provided only a lower rate limit to prevent life-threatening asystole. Thereafter, more complex pacemakers were designed to respond to sensed electrical conditions in the heart, providing a more physiologic response. For example, if a patient's atrium were producing a relatively normal and physiologically appropriate heart rate, but the signal was not propagating into the ventricle, an appropriate pacemaker might sense atrial beats and pace the ventricle in synchrony. More recently, pacemakers have been proposed which sense environmental or physiologic changes indicative of greater demand, and adjust the allowable minimum heart rate accordingly. An example of an environmental parameter detected by a pacemaker was disclosed by Dahl in U.S. Pat. No. 4,140,132. In that patent, acceleration, which correlates to the activity of the user, was used to adjust the minimum pacing rate. Other so-called rate responsive pacemakers have sensed impedance, pH, O.sub.2 and other factors to provide an appropriate physiologic pacing rate.
In addition to the minimum rate, other parameters may be changed to optimize cardiac output. These parameters include refractory periods, stroke volume, sensing amplifier thresholds and stimulating pulse adjustments. To adjust such parameters, pacemakers have been proposed to perform various tests. In other words, the pacemaker produces some signal and detects the heart's response, thereafter adjusting its next signal to the form of the cardiac response. My invention seeks to optimize pacemaker function by initiating such test sequences when sensed physiologic or environmental changes indicate the probability of a change in non-rate parameters.
The magnitude of the output pulse is a non-rate parameter which could be tested and adjusted from time to time when environmental or physiological changes have been sensed. There exists a certain minimum energy necessary to stimulate the muscles of the heart. This energy is delivered by applying a given current or voltage for a particular duration through an electrode. The voltage or current necessary for stimulation is inversely related to the duration of the impulse. This relationship varies from patient to patient and from time to time for the same patient. However, the so-called Lapicque equation describes generally the behavior of the voltage or current with respect to the impulse duration. The equation is: ##EQU1##
In this equation Y is the pulse amplitude (either voltage or current); Y.sub..infin. is the rheobase or minimum amplitude required to produce a stimulated response as pulse duration goes to infinity; T.sub.c is the chronaxie or minimum duration necessary to achieve stimulation at twice the rheobase; and T is the pulse duration. In the absence of other factors, a pacemaker would last longest if its output pulse expended the minimum energy, a condition which occurs at an impulse duration equal to the geometric mean of the current chronaxie and voltage chronaxie. For further explanation of this phenomenon see, e.g., Bernstein, et al., "Threshold Curve Approximations for Pacemaker Output Programming", Proceedings of the 2nd European Symposium on Cardiac Pacing, G. A. Feruglio (ed.), Piccin Medical Books, 1982, pp. 115-120.
In practice, however, the theoretical minimum energy is not used. Because it is critically important that capture be achieved, that is, that the heart be stimulated to contract, a certain safety factor is employed to increase the magnitude of the pulse. Moreover, it is known that the stimulation threshold varies over time after implantation of a new lead. This phenomenon may be associated with the reaction of the body to a foreign body in the heart. Physicians, therefore, compensate for this increased resistance to stimulation.
In addition, researchers have found that the stimulation threshold may change in a given patient in response to increased heart rate, changed physical or emotional stress, or other factors. Various conditions correlated to these factors have, in the past, been sensed in order to vary the pacing rate. These conditions include acceleration, vibration, pH, O.sub.2, impedance, and the levels of catecholamines, for example, the level of epinephrine. Such detected changes have not been used, however, to indicate potential variation in the rheobase or other parameters related to the output pulse and to initiate a search for the optimum output level, whereby the energy of the pacemaker can be conserved, and its longevity improved.
Similarly, tests for changes in other parameters such as refractory periods, stroke volume, or amplifier thresholds, have not been conducted in response to sensed changes in physiologic or environmental conditions.